It is common in many fields to use lifting equipment consisting of a tubular with one end attached to a winch mechanism to disperse and to retrieve the tubular. The opposing end of the tubular is attached to an object to be lifted. In the context of this disclosure, “tubular” refers to any axial structure which can be coiled around a winch, including cables, wires, belts, hollow tubes of cylindrical or other shapes, ropes and other cordage, and the like. As an example, cranes utilizing a winch and a metal cable are used in construction to hoist beams, concrete, roofing material and other construction material as a structure is being built. As another example, many types of winch-cable lifting devices employ metal cable, rope or other cordage to load and unload cargo. As yet another example, “draw works” consisting of a lifting mechanism and various tubulars attached thereto are used in hydrocarbon production to drill well boreholes, to dispose equipment within the boreholes, and to convey equipment within the borehole to measure properties of material penetrated by the borehole.
From operational, maintenance and safety aspects, it is usually desirable to measure tension in the tubular attached to the winch. Operationally, these devices have a lifting limit therefore a measure of tubular tension is useful in remaining within limits of the device. From a maintenance aspect, abnormal tubular tension often is an indication of an equipment maintenance problem. From a safety aspect, excess tubular tension can result in cable breakage with risk to human life and physical surroundings.
It is usually desirable to measure tension both when the tubular is stationary and when the cable is axially moving due to the action of the winch. Apparatus to measure axial tension is referred  to as a tensiometer. One type of tensiometer employs at least one sheave. Stated simply, a sheave is a device that changes axial direction of a cable, wire or any other type of tubular that passes over the sheave. The most common form of sheave is a circular “wheel” with a groove in the outer perimeter of the wheel to receive the tubular. Tensiometers can employ sheaves in a variety of embodiments. Tensiometers can comprise a single sheave, or one or more measurement sheaves cooperating with one or more guide sheaves. As an example, a tensiometer can comprise a measurement sheave wheel and first and second guide sheave wheels disposed on opposing sides of the measurement sheave. This type of tensiometer will be illustrated and discussed in more detail in a subsequent section of this disclosure, and will be used as an example to illustrate basic principles applicable to other embodiments of sheave type tensiometers. Briefly, the tubular enters the tensiometer, passes over a first guide sheave wheel, is deflected from its original path when passing over the measurement sheave wheel, and is returned to its original path when passing over a second guide sheave. The deflected tubular exerts a force on the measurement sheave wheel which is typically perpendicular to the original path of the tubular. A measure of this force can be related to axial tension in the tubular.
Effective diameters and relative positioning of measurement and guide sheave wheels in all embodiments of sheave tensiometers affect the precision of the tension measurement. The term “wrap” is defined as an arc in which the tubular contacts a sheave wheel. In general, a greater deflection of the tubular results in a more precise measurement of tension. Stated another way, resolution and stress on retaining hardware of the tensiometer increase as the angle of wrap increases. Tubular bending stress is inversely proportional to the radius of bend when the tubular is deflected. Bending stress does not, however, increase with the angle of wrap. For a given angle of deflection (therefore a given measurement precision), bending can be lessened by increasing the diameters of the sheave wheels. It is, therefore, desirable for the measurement and guide sheaves to be as large in diameter as possible while still meeting other dimensional restrictions of the tensiometer. Unfortunately, space on most lifting devices is usually limited therefore forcing a compromise in selecting a tensiometer between measurement precision and size. 